Increasing the concentration of a reactant in solution increases the rate of reaction.Concentration is the amount of a substance in a given volume, so a higher concentration means more reactant particles per unit volume (per dm3). The particles are closer together, so they collide more often. The energy of the particles is unchanged, so the proportion of collisions with energy greater than or equal to the activation energy stays the same. However, because there are more collisions per second in total, there are more successful collisions per unit time, and the rate increases.For example, magnesium ribbon reacts faster with 2.0 mol dm−3 hydrochloric acid than with 1.0 mol dm−3 acid:Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) Hydrogen is given off more quickly in the more concentrated acid.
As a reaction proceeds, the reactants are used up, so their concentrations fall. This is why the rate of most reactions is greatest at the start and decreases over time. A graph of volume of gas against time has its steepest gradient at the beginning and levels off when one reactant has been used up.In exam answers, say that concentration changes the frequency of collisions, not the energy of each collision. Do not say that the particles move faster.
As a reaction proceeds, the reactants are used up, so their concentrations fall. This is why the rate of most reactions is greatest at the start and decreases over time. A graph of volume of gas against time has its steepest gradient at the beginning and levels off when one reactant has been used up.In exam answers, say that concentration changes the frequency of collisions, not the energy of each collision. Do not say that the particles move faster.